Developing non-invasive blood-based tests is extremely appealing for presymptomatic screening and early detection of cancers where obtaining tissue biopsy is highly invasive, difficult, or costly, such as lung cancer and ovarian cancer. This is particularly true for many primary tumors and most metastatic diseases. Probing circulating exosomes is an emerging paradigm for non-invasive cancer diagnosis and monitoring of treatment. Most eukaryotic cells release numerous membrane-derived vesicles into extracellular environment which are mainly composed of exosomes (30-150 nm) and microvesicles (200-1000 nm) differing in their cellular origin, abundance and biogenesis. Exosomes are commonly recognized as membrane vesicles derived from the endolysosomal pathway with a size range of approximately 30-150 nm, and are abundantly found in the plasma and malignant effusions derived from cancer patients. Exosomes share certain common characteristics, including shape, size, density, and general protein composition, and mediate effects via transfer of cargo consisting of an array of proteins, selected functional cellular RNAs, and mitochondrial DNA. Despite the potential for cancer research, exosome analyses have been severely constrained by the daunting technical difficulties in isolation and molecular analysis of such nano-scale and molecularly diverse vesicles. Current isolation protocols largely rely on multiple ultracentrifugation steps, which are tedious, time-consuming (>10 h) and inefficient. Moreover, differential ultracentrifugation co-purifies several microvesicle subpopulations which are secreted by different intracellular mechanisms, and thus could potentially mask the disease-related biosignature. Size exclusion methods normally do not concentrate exosomes and are prone to pressure-caused damage of vesicles and contaminations. Molecular analysis of isolated microvesicles is primarily performed using Western blot, ELISA and mass spectrometry, which require lengthy processes and large sample sizes (purified from more than 3 mL plasma or 300 mL cell culture media), limiting the progress in clinical investigation and utilities of exosomes. To date, no well-defined protocols and markers for quantitative evaluation of extracellular vesicle proteins exist.
Microfluidic technology has shown unique advantages in bioassays, such as ultrahigh throughput, single molecule/cell sensitivity and resolution, multifunctional integration, and automated operation with minimal sample consumption, to facilitate quantitative biology and medicine. Although recent advance of microfluidic technology has made an impact on many areas of biology, there is an ongoing need for microfluidic technology to accelerate exosome analysis.